Grundfos MQ 3-35: Short Cycling Issue – Flow Sensor & Control Unit Replaced, Problem Persists – Suggestions?

[DaanDS]

Hi all,

I’ve been going through various topics here but haven’t found an exact match for the issue I’m experiencing.

My pump has started short cycling.
After reading through some similar cases, I’ve tried a few solutions, but the problem persists.

Here’s what’s happening:

• When I open a water outlet, the pump runs but short cycles after about 12 seconds. Occasionally the short-cycle does not happen.
• I replaced the flow sensor, but that didn’t resolve the issue.
• I swapped the pump controller with one from an MQ3-45 pump. Initially, this seemed to fix the issue, but after approximately 20 on/off cycles, the problem returned.

Currently, the pump behaves as follows:

• It turns on when there’s a water demand, though not immediately—there’s a brief delay until some milliliters of water flow.
  • Therefor the internal pressure tank appears to operate as expected.
• First +- 20 open/close cycles after changing the control unit seems to behave as expected, pump will run +-10 seconds after closing water demand, as described in manual.
• The pump also shuts off when water demand stops.
• Short cycle occures 95% of the time
• Water pressure seems fine
• Water flow rate seems fine

I’ve cleaned the pump filters behind the pump, and there doesn’t seem to be any obstruction causing the issue.
The non-return valve in the water well doesn’t seem to be an obvious issue at first glance. I’m not sure how to properly test it.


Does anyone have any advice or suggestions on what might be causing this?

Thanks in advance!

 

[Valveman]

Sorry for your problems. A search in this forum for "MQ" brings up at least 8 pages of threads from people with similar problems. It all boils down to the same thing. Those MQ pumps were not made to last. How could any self promoting pump manufacturer make a pump that would last and stay in business? The MQ is just one of many "tankless, all in one, suppose to make your life easier, disposable pumps". They are made to sound cool, be easy to install, and be replaced often, to grease the wheels of the big pump industry. They are not made to be repaired and very few people try it. They made the Scala to replace the MQ. But the Scala has an even worse design life, so they didn't discontinue the MQ.

If you want something that will last and not give any trouble for 30-50 years, you need a regular jet pump with a regular pressure switch, Cycle Stop Valve, and small pressure tank. This design has proven to make pumps last a lifetime, which is why you will not hear any pump manufacturer ever mention a CSV.

The first picture below shows how to make a long lasting constant pressure pump system.
The second picture shows a jet pump with a predecessor of the Cycle Stop Valve that lasted more than 50 years.
The third picture shows a dumpster full of MQ pumps, which is what I see behind all the pump supply houses.

 

[Sponsor]

 

[DaanDS]

Thanks, @Valveman, for taking tour time to respond.

I’ve gone through other posts on the Terrylove forum and found plenty of similar issues.
However, I didn’t come across any discussions about the problem persisting even after replacing the controller unit and flow sensor.

While I agree that these models aren’t built to last, I’m curious - if the issue isn’t with the flow sensor or control unit, what could be causing it to persist?

Out of interest, and in an effort to get this unit working again (even if only temporarily), it would buy me some time to transition to a jet pump as mentioned. If you would have any European backed suggestion to brand for the Jet pump and/or pump control kit, i'm all ears!


Thanks again,
Great evening!

 

[Valveman]

After re-reading, if the pump comes on when no taps are open the one way check valve is bad. They can work intermittently and be giving that problem. But if a tap is open when the cycling happens it is not the fault of the check valve. The MQ starts when the flow switch moves up or the pressure falls to like 27 PSI. But there is no pressure shut off, only flow switch. So, if it shuts off at all the flow switch is working. The control board is supposed to time out for 10 seconds after the flow switch drops before shutting off the pump. But it is an electronic control board so it could be glitching the way any computer will.

 

[rpo1728]

Sorry for your problems. A search in this forum for "MQ" brings up at least 8 pages of threads from people with similar problems. It all boils down to the same thing. Those MQ pumps were not made to last. How could any self promoting pump manufacturer make a pump that would last and stay in business? The MQ is just one of many "tankless, all in one, suppose to make your life easier, disposable pumps". They are made to sound cool, be easy to install, and be replaced often, to grease the wheels of the big pump industry. They are not made to be repaired and very few people try it. They made the Scala to replace the MQ. But the Scala has an even worse design life, so they didn't discontinue the MQ.

If you want something that will last and not give any trouble for 30-50 years, you need a regular jet pump with a regular pressure switch, Cycle Stop Valve, and small pressure tank. This design has proven to make pumps last a lifetime, which is why you will not hear any pump manufacturer ever mention a CSV.

The first picture below shows how to make a long lasting constant pressure pump system.
The second picture shows a jet pump with a predecessor of the Cycle Stop Valve that lasted more than 50 years.
The third picture shows a dumpster full of MQ pumps, which is what I see behind all the pump supply houses.

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Hey you seem to really understand these guys and it's nearly impossible to get tech support from grundfos. Another tech was out recently and changed the pressure gauge on the discharge line. Since then they've been having pressure issues. I watch the pressure drop to about 10 psi before the pump will kick on. When it does it gets up to 70 and stays there. This happens constantly. If I unplug pump while water and pump is running and plug it back in, it'll kick on. But again if the pump satisfies, turns off and I run water it will drop to 10 psi before it kicks on. What do you think ? Thanks in advance

 

[Valveman]

Hey you seem to really understand these guys and it's nearly impossible to get tech support from grundfos. Another tech was out recently and changed the pressure gauge on the discharge line. Since then they've been having pressure issues. I watch the pressure drop to about 10 psi before the pump will kick on. When it does it gets up to 70 and stays there. This happens constantly. If I unplug pump while water and pump is running and plug it back in, it'll kick on. But again if the pump satisfies, turns off and I run water it will drop to 10 psi before it kicks on. What do you think ? Thanks in advance

Sorry for the delay. Those MQ pumps have a flow switch and a pressure switch. There is also a little pressure tank inside there somewhere. When you open a tap the flow from the tablespoon size pressure tank is supposed to lift the flow switch and start the pump. But when the demand is small, like a leak, the flow is not enough to lift the flow switch. So, there is also a preset pressure switch that will start the pump when it gets down to like 17 PSI is I remember correctly.

I would guess the little pressure tank has failed and it must drop to low pressure before it will start each time. It could also just be a glitch in the electronics. Electronics, flow switches, and built-in glitches are how they keep the wheels of big pump businesses going round and round. Somewhere I have a picture of a regular dumb pump that lasted 50 something years with a regular pressure switch/pressure tank, and a anti-cycling valve similar to a Cycle Stop Valve.